So, I don't know if I will be heard on this thread but after a long hesitation, grudgingly I installed Safari 9.1.3 update on Mavericks but feel like choosing the lesser of two evils, instead of feeling relieved. Prior to that I lived torn apart between performance of the OS as a whole and Safari 7's failures to load some content properly and by that I mean sites like Vimeo, Stackexchange, Stackoverflow, Flickr, Twitter, Reddit: it was just like sitting on 2 chairs and constantly feeling psychologic discomfort. Interesting that Safari 7.1.8 released in 2015 struggled while Safari 9.1.3 released after 13 months could load these sites, YouTube looks more up-to-date with Safari 9 and closer to what Chrome 67 and Waterfox output. I noticed such thing with Chrome 49 on Lion which is a 2016 release too. Projecting from this day back, 2016 is the bottom edge for the modern and more or less satisfying browsing experience. I now use other browsers less and for quick switching I bought "Browser Fairy" 2 months ago on MAS for use on older systems.
Yes, the new WebInspector is
awful and was the main reason to hold out. I couldn't but curse all the time at it: why couldn't they just leave its UI alone, why?!! Damn! It's so ugly and messy, grey, dark, primitive..ugh
At least extensions work as before.
P.S. Experimenting and out of curiosity I headed for Steam site I saw on your screenshot: Safari 9 loads only a textual version automatically. Just like Chrome 49 on Lion Safari 9 is able to pass authorization on Apple Support Communities but is incompatible with basic actions (posting, selecting topics etc.) despite the fact that it was released later than Chrome 49. Apple Developer Site rejects it completely ("unable to establish secure connection").
At least I updated Mavericks one step closer to the present day.